The present invention relates to a method of driving a liquid crystal matrix display panel, and is particularly directed towards a drive method for a liquid crystal matrix display panel having a large number of display elements, suitable for use as a display terminal in data processing equipment. Such a display is utilized for patterns which represent characters, numerals or graphics (e.g. charts, etc) and which are generally held static on the display screen, or move relatively slowly across the screen. Thus, the patterns produced by such a display will in general remain static during a large number of successive frame intervals (with all of the rows of elements of the display being successively scanned during each frame interval). For ease of description, it will be assumed in the following that each display element of a liquid crystal matrix display panel can attain only an ON and an OFF state, and that the conductors connected to respective rows of display elements, which are successively scanned by drive signal pulses of fixed amplitude (generally referred to as common drive signals), are aligned horizontally and will be referred to as common conductors, while the vertically arrayed conductors which are connected to respective columns of display elements and are driven by data-dependent signals (generally referred to as segment drive signals) will be referred to as segment conductors. It will also be assumed that the display is of the type in which a display element is set in the ON state, to appear dark in color against a background of light-colored OFF state display elements, by application of an RMS level of voltage to the display element of sufficiently high value. The invention is however not limited to liquid crystal displays of the latter type. All of the rows of display elements are successively scanned by the common drive signals during each of successive frame intervals.
As the number of display elements of a liquid crystal matrix display panel is increased, it is found that the display quality deteriorates. Specifically, a reduction of contrast occurs, i.e. the most completely "black" level of display cannot be attained. This is due to various factors, the most important of which are the effects of increased resistance of the conductors which supply drive signals to the display elements, as the size of the display matrix is increased, in conjunction with increased display element capacitance which must be charged and discharged by drive signals applied over those conductors, together with reduction of the duty o5 ratio for which each display element is driven. The present invention is directed towards a further problem which has arisen in recent years with the development of liquid crystal matrix display panels having a large area and a very high display element density, e.g. having 100 rows of display elements or more. This problem is manifested as display contrast irregularity, i.e. the coloration of dark-state and light-state areas of the display is not uniform over the entire display, but is pattern-dependent. For example, in a column of display elements containing a number of successively adjacent display elements which are all driven to the ON (i.e. dark) state, the degree of dark-state density attained by the ON state display elements will be higher (and the OFF state display elements will appear darker) than in the case of an ON state display element in a column in which a number of display elements are successively set in the ON and OFF states in an alternating manner. This effect is due to the fact that, although each display element is periodically addressed to be driven into the ON state or OFF state by voltages applied simultaneously to the corresponding X and Y-direction conductors, the effective RMS value of drive voltage applied to a display element will be affected by the states of other display elements driven by the same common conductor. For each display element, during the nonselection portion of each frame interval (i.e. all of the frame interval other than the portion in which that display element is addressed), a drive signal will be applied which will vary in waveform in accordance with the display states of the other display elements in the same column. If this drive signal contains a substantial high-frequency component then this will be effectively blocked by the resistive impedance of the long, narrow and transparent (hence extremely thin) drive conductors, in combination with the capacitances of the display elements, and so does not significantly affect the effective drive voltage applied to each display element of that column. However if the drive signal contains a large low frequency component, then this will be less affected by the latter resistance-capacitance blocking effect, and will result in a higher RMS drive voltage being applied to each display element driven by that segment conductor. As a result very conspicuous effects, such as vertical stripes of varying density in the (light color) background areas will appear on the display, which will move in accordance with changes in the display pattern.
This problem of pattern-dependent display contrast variation is increased as the display element density and the number of display elements is increased, since the increased display element density will necessitate reduction of drive conductor cross-sectional area and hence increased conductor resistance, while resistance will be further increased by the greater lengths of the common conductors and segment conductors as the number of display elements in the display is increased, while the amount of display element capacitance connected to each drive conductor will also increase proportionately.
Methods of overcoming this problem have been proposed hitherto, as described hereinafter, but these have only proven partially successful. One such proposal has been made in a paper entitled "SID Japan Display '83--the 3d International Display Research Conference Post-Deadline Papers PD5". However as described hereinafter, this proposed method is not effective for all display patterns.